The present invention relates to a wafer holding chuck useful for supporting and positioning a wafer during manufacturing or testing thereof. Previously known wafer holding chucks have taken numerous forms. In the simplest case, a flat surface is provided with an aperture at the center to which a vacuum source is applied. A wafer can then be placed on the wafer supporting surface and the vacuum will serve to hold the wafer on the chuck securely. A variation on this design is a chuck having a plurality of openings dispersed around the wafer holding surface so as to provide multiple vacuum ports for more securely holding a wafer to the chuck. Other designs have also been utilized which include one or more grooves cut in the surface of the wafer holding chuck with one or more vacuum ports being formed at the bottom of the grove so as to provide a method for widely distributing the vacuum port when a wafer is placed on the chuck.
The designs heretofore known, however, have not proved to be entirely satisfactory for the purpose of holding wafers and particularly semiconductor wafers during either manufacturing or testing steps. One particular problem frequently encountered with known vacuum chucks is that in some instances they simply do not hold the wafer on the chuck very well. This is particularly noticeable in wafer holding chucks where there are a small number of vacuum ports in the surface of the chuck. In such situations, it has frequently been observed that the wafer simply is not held well enough so as to prevent movement thereof as a result of vibration, shaking or moving the chuck itself.
In overcoming the above difficulty, in some instances manufacturers have suggested using stronger vacuum sources. This in and of itself adds to the cost but it also has a tendency to warp wafers which themselves may be slightly warped when put on the wafer chuck. Accordingly, the cost and the difficulty of adjustment presents further objections to the use of some known designs.
A further difficulty encountered with prior art wafer holding chucks is that adjustment of the operation thereof is not easily accomplished so as to provide the maximum holding power when the largest wafer is mounted on the chuck while having reduced holding power when a small wafer is to be held by the chuck. The inability to adjust the vacuum easily results in warping the wafer being held thereby in many situations. Such warping results in the features on the wafer being stressed which may produce electrical faults in the circuits on the wafer.